This is a continuing patient-oriented research study of congenital hyperinsulinism (HI), the most frequent and most difficult cause of hypoglycemia in infants and children. Four genes have been associated with HI: recessive, loss of function mutations of the two KATP channel genes (SUR1 & KIR6.2) and dominant, gain of function mutations of glutamate dehydrogenase (GDH) and islet glucokinase (GK). KATP defects can also cause focal HI through loss of heterozygosity for the maternal allele leading to expression of a paternally-derived mutation. We have recently identified dominantly-expressed cases of SUR1 mutations. Since the 4 known HI loci were found by positional cloning, methods are lacking for their clinical diagnosis and additional HI genes are likely. The goal of this grant is to define the genetic determinants of the three most important clinical features of HI: protein sensitive hypoglycemia, diazoxide responsiveness vs non-responsiveness, and focal vs diffuse pancreatic lesions. Aim 1 is to determine the glucose, insulin, and gluco-regulatory hormone responses to protein tolerance tests in children with different forms of HI. Studies will test if protein sensitivity occurs in other forms of HI besides GDH-HI and how dysregulation of other hormones contribute to hypoglycemia. Aim 2 will use acute insulin response (AIR) tests and mutation analysis to identify the causes of diazoxide-responsive HI. Our hypothesis is that most cases will have recessive or dominant KATP mutations capable of retaining partial function. Aim 3 is to evaluate new methods for pre-operative diagnosis of focal HI using functional testing and rapid genetic analysis to detect single, paternal KATP mutations. Aim 4 is to identify the molecular defects in children without mutations in known HI genes by screening for non-coding defects of the KATP genes and screening of novel candidate genes. The results of this research will provide essential information for improving the diagnosis and treatment of HI children and for understanding the basis of insulin regulation in normal humans.